Improved aerodynamics for motor vehicles are leading to future windshield designs with more pronounced rake angles, which produce a larger window surface. A window wiper system for such windshields must therefore include longer, more massive wiper arms and blades to wipe the required percentage of the larger surface. In a symmetrical overlap wipe pattern, in which the blades are oscillated in opposing movement between separated outer wipe positions and an overlapping inner wipe arrangement, longer arms and blades produce a larger overlap or collision avoidance region and, consequently, a need for increased blade separation.
Present symmetrical overlap wiper systems use a single motor with an unbalanced linkage to move the wiper blades through a repeating pattern, with one of the blades always overlapping the other in an overlapping inner wipe arrangement. The overlapping blade leads the other from the overlapping inner wipe arrangement through the overlap region toward its outer wipe position and follows the other back through the overlap region into the overlapping inner wipe arrangement. It must therefore pause at its outer wipe position to allow the other blade time to reach its outer wipe position and return to precede it back into the overlap region. When the paused blade is restarted, it must be rapidly accelerated to make up for the time lost in the pause in order to achieve high wipe rates such as 75 wipes per minute; and this rapid acceleration of a large mass puts extra stress on the linkage components and may lead to an undesired whipping action of the blade ends.
A variation of the symmetrical overlap pattern described above is the alternating symmetrical overlap pattern, in which the upper blade in the overlapping inner wipe arrangement is alternated in each cycle. In this pattern, the first blade out no longer has to wait while the other blade reaches its outer wipe position but starts back without pause, providing, as will normally be the case, that the other blade has cleared the overlap region. With no requirement for a long pause in the outer wipe position, the blade does not need to be accelerated to such a great degree in its return. The alternating symmetrical overlap system of this invention thus significantly reduces stress on arms, blades and linkages and reduces blade end whip at high wipe cycle rates.
However, in an alternating symmetrical overlap wiper system, the leading wiper blade moving outward to its outer wipe position may be slowed down by different motor speed or glass conditions so that the following blade is the first to reach an outer wipe position. If this occurs, the wipe cycle may be completed sooner by allowing this following blade to immediately reverse and precede the slowed blade back toward the overlapping inner wipe arrangement.